Thermal insulation materials

ABSTRACT

A thermal insulating material comprising a double-faced knitted glass fibre fabric in which the faces (16, 17) of the fabric are interconnected by at least one linking thread (18) which passes from one face (16) to the other (17).

FIELD OF THE INVENTION

This invention relates to thermal insulation materials and to a methodof manufacturing such materials.

BACKGROUND OF THE INVENTION

There is a need for a lightweight flexible sheet material which has lowthermal conductivity, but which can be fabricated into thermalinsulation blankets or panels. Ideally such flexible sheet materialsshould be safe to use and not produce dust or fibre particles which canbe inhaled or cause irritation to the skin of anyone who comes intocontact with the material. There are some applications which requiresuch sheet material to be re-useable many times.

In some applications, the material has to withstand exposure to veryhigh temperatures and also provide a thermal insulation barrier, andthere are few materials which possess both resistance to hightemperature and low thermal conductivity.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided aflexible thermal insulating fabric comprising a double-faced weftknitted structure formed by knitting yarn which comprises strands ofair-textured glass fibre to produce two spaced knitted faces interlinkedby yarn which passes from one knitted face to the other.

In a further aspect of the present invention there is provided a methodof making a flexible thermal insulation fabric comprising the steps ofweft knitting a double faced glass fibre fabric using yarn whichcomprises strands of air-textured glass fibre on a double needle bedweft knitting machine and interconnecting the faces of the fabric withat least one linking yarn which passes from one knitted face to theother. The or each linking yarn may be formed by tuck stitches whichpass from one face of the fabric to the other.

In a preferred embodiment of the present invention, the thermalinsulation material is knitted on a double needle bed weft knittingmachine which uses a "V" bed with 2.5 gauge needles.

The spacing between the front bed needles and the back bed needles issuitably about 10 mm, and this dimension affects the overall thicknessof the finished fabric as will be explained below. If desired thespacing between the front and back needle beds could be greater than 10mm if thicker fabrics are required.

Preferably linking yarn in the form of tuck stitches are created bywrapping the at least one linking yarn around selected needles of bothneedle beds.

Preferably the or each linking yarn is a glass fibre thread.

In a preferred embodiment of the invention glass fibre threads areconverted to silica by leaching the fabric in an aqueous solutioncontaining hydrochloric acid.

In yet a further embodiment of the invention a leached fabric has afinish applied to at least one of the faces. The preferred finish isapplied by immersing the fabric in a solution comprising 50% by weightvinylacetate ethylene copolymer latex and an aqueous silicone elastomeremulsion

The preferred yarn for knitting comprises a plurality of strands ofair-textured glass fibre (each of which is about 1700 decitex) fed to ayarn feeder of the knitting machine.

Preferably the thermal conductivity of the fabric, measured in adirection normal to both faces, is of the order of 0.01 to 0.20 w/m°k.Ideally the thermal conductivity is in the range of 0.10 to 0.125 w/m°k.

In one embodiment of the invention, the thermal insulation material maycomprise a first substantially silica fabric joined to a second glassfibre fabric.

In a further embodiment of the invention the thermal insulation materialmay comprise a core fabric made of glass fibre and a silica fabricjoined to the surfaces of the core fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be further described, by way of example,with reference to the accompanying drawings in which:

FIGS. 1 to 5 illustrate schematically the stitch patterns for knittingfive thermal insulation materials in accordance with the presentinvention, and

FIGS. 6 to 8 show schematically the cross-section of three materialsmade in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In all of the following examples, the thermal insulation materialcomprises a knitted fabric which has two knitted faces spaced apart in adirection along which heat, which is to be shielded by the fabric,flows. The two spaced faces are interconnected by stitches which passfrom one face to the other so as to constitute a unitary body which hasa low density (due to the presence of a large volume of air trappedbetween the two faces). The low density core so formed is substantiallyself supporting, that is to say that the two faces of the fabric, whilstable to be displaced if moved relative to each other by small amounts indirections parallel to the faces, are nevertheless tied together as aunitary body by the interlinking stitches so that the body issubstantially self supporting.

Referring to the stitch pattern diagram of FIG. 1, a first course isknitted on all the needles 10 of the front bed of needles (stage (a)).

A second course is then knitted on all the needles 12 of the back bed ofneedles (stage (b)). The third course is formed by wrapping the yarnaround the needles 10 of the front bed across the gap between the frontand back needle beds and around the needles 12 of the back bed (stage(c)).

This three-course pattern is then repeated until the desired length offabric is produced. The resulting fabric comprises two fabric facesinterconnected by the tuck stitches formed by each third course of therepeated pattern.

The overall thickness of the fabric is dependent upon the distancebetween the needles of the front bed and the needles of the back bed,the gauge of the needles and the tension of the yarn used to make thetuck stitches in each third course.

The typical weight of a fabric made in accordance with the stitchpattern illustrated in FIG. 1 is about 3 kg per square meter, and thefabric has a thickness of about 13 mm. The thermal conductivity istypically 0.125 w/m°k, measured in the direction normal to both faces.

In the above-described stitch pattern, the third course is wound aroundall the needles of each needle bed. If desired, the thread may be woundaround only some of the needles of each bed as shown in course (c) ofFIG. 2. This has the advantage of reducing the total weight of thefabric for a given thickness. Referring to FIG. 2, the same thicknessglass fibre yarn is used as that used in FIG. 1 and the first twocourses are knitted exactly as described with reference to FIG. 1.

In a further embodiment of the present invention the stitch patternshown in FIG. 3 is used. The first and second courses are knitted asdescribed above with reference to stages (a) and (b) of FIG. 1. A thirdcourse is formed by wrapping the thread from alternate needles 10 of thefront needle bed to alternate needles 12 of the back bed as shown in3(c). The pattern is repeated except that the sixth course is formed bywrapping the interlinking thread from the alternate needles 11 of thefront bed to the alternate needles 13 of the back bed as shown in 3(f).If desired, different thickness yarns may be used for the third andsixth courses.

In yet a further embodiment of stitch pattern shown in FIG. 4, a doublezig-zag tuck stitch pattern can be achieved by knitting the first twocourses as described in connection with FIG. 1, but forming the thirdcourse by wrapping interlinking thread around alternate needles 10 ofthe front bed and around the alternate needles 12 of the back bed asshown in 4(c). A fourth course is formed by wrapping the same or adifferent interlinking thread around the alternate needles 11 of thefront bed and the alternate needles 13 of the back bed as shown in 4(d).The pattern of these four courses is then repeated until the desiredlength of fabric is produced.

In yet a further embodiment shown in FIG. 5, one face F of the fabric isknitted on 5 gauge needles 14 and the other face B of the fabric isknitted on 2.5 gauge needles 15.

Referring to FIG. 5, the first course is knitted on all the back bedneedles 15 using a glass fibre yarn comprising five threads, each of1700 decitex as shown in FIG. 5(a). The second course is knitted on allthe needles 14 of the front bed using two strands of 1700 decitex glassfibre as shown in FIG. 5(b).

The third course is formed by wrapping a thread of glass fibre,comprising two strands of 1700 decitex glass fibre, around all theneedles 15 of the back bed and alternate needles 14 of the front bed asshown in FIG. 5(c).

The resultant fabric has the one face F which is of relatively tightknitted stitches knitted on the smaller (5 gauge) needles 14 and theother face B exhibits relatively loose stitches, knitted on the largerneedles 15. The tight knitted face F may provide a better surface forsubsequent coatings (as described hereinafter) than the loose knit faceB.

All of the materials produced as described above with reference to FIGS.1 to 5 comprise two faces 16,17 (shown in FIG. 6) linked together bytuck stitches 18 formed by wrapping the glass fibre thread aroundselected needles of both beds as described above. The resultingmaterials have low thermal conductivity and, because of the uniquecombination of the needle size, thickness of yarn, and tension of theyarn, are lightweight and very flexible and safe to handle. All theproducts produced as described above offer effective thermal insulationfor low temperature application (up to for example 700° C). However, theglass fibres will soften or melt at about 700° C. so, if the product isrequired to withstand exposure to heat at temperatures above 700° C., itis necessary to apply further coatings to at least that surface of thefabric exposed to the high temperature.

In one embodiment, a coating comprising a refractory material such as avermiculite slurry is applied to one or both faces of the fabric. Inanother embodiment a perfluorocarbon such as PTFE may be applied to oneor both surfaces.

In yet a further embodiment of the present invention the knitted fabric,produced as described above (other than that it has a vermiculitecoating applied to it), is leached by immersing the fabric in a leachantwhich comprises hydrochloric acid in order to convert the glass fibre tosilica. A fabric made by the method of FIG. 1, which started at 13 mmthickness before leaching, reduces to about 10 mm overall thicknessafter leaching. Approximately 98% of the glass is converted to silica.The leached fabric still retains its flexibility but will withstandexposure to temperatures of up to 1600° C. before the silica melts. Thethermal conductivity of the leached fabric is of the order of 0.10 w/m°k.

In a preferred embodiment, the leached fabric has a finish applied to atleast both faces of the fabric in order to provide abrasion resistanceand to suppress the creation of dust. A preferred method of applying thefinish comprises the steps of immersing the leached fabric in a finishsolution comprising 50% by weight vinylacetate ethylene copolymer latex(an example being that sold under the trade mark VINAMUL 3237) and anaqueous silicone elastomer emulsion (an example being that sold underthe trade mark ULTRATEX FSB).

Referring to FIG. 7 there is shown, schematically, a thermal insulationmaterial constructed in accordance with the present invention. Thematerial is suitable for use as a thermal insulation blanket that can bewrapped around a component such as a pipe.

The material comprises an unleached fabric 20 manufactured as describedabove with reference to any one of FIGS. 1 to 5 and a leached fabric 21manufactured as described above with reference to any one of FIGS. 1 to5, leached in aqueous hydrochloric acid to convert the glass fibre tosilica as described above and coated with a finish by immersing in thefinish solution described above.

The fabric 20 is secured to the fabric 21 by stitching, stapling or bymeans of an adhesive so as to form a unitary body which is flexible.Such a body has the ability to withstand high temperatures because ofthe layer 21 and possesses low thermal conductivity because the layer 20is a low density fabric with many voids formed within the fabric.

If desired, a unitary body could be made comprising an unleached corefabric 20 (made as described above) clad on both sides with a leachedfabric 21 (made as described above). An example of such a fabric isshown in FIG. 8.

In the above examples, the leaching of the glass fibres to form silicais carried out by immersing the whole fabric destined to form the layer21 in the leachant.

In the above examples the thickness of the fabric is determined by thewidth of the gap between the needle beds. Conventional V-bed weftknitting machines can be adapted to be used to make fabrics inaccordance with the present invention. The common practice withconventional V-bed machines is to design the shape of the cams whichcontrol the throw, or movement of the needles so that after the needlesare pulled to a maximum position when forming the loops on the needlesthey are backed-off a small amount to release tension so as to avoidbreaking the thread. In the context of the present invention, it isdesired to produce the thickest possible fabric (for thermal insulationreasons) and backing off the needles to relax tension would not optimisethe thickness of the fabric. Therefore, it is contemplated that the camsof a conventional V-bed machine could be modified so as to reduce, orpossibly eliminate, the amount that the needles are backed off torelieve tension. Such a design modification would be unusual forknitting textile fabrics and for most glass fibre fabrics would be anunnecessary and unneeded expense. However, for the purposes of thepresent invention, one can achieve slightly thicker thermal insulatingfabrics for a given gap between needle beds by not backing off theneedles, than one can achieve when backing off the needles.Surprisingly, this has been achieved without breaking the glass fibreinterlinking threads, which in any case are relatively thicker than themore usual glass fibre threads used for fabrics.

We claim:
 1. A flexible thermal insulating fabric comprising adouble-faced weft knitted structure formed by knitting yarn whichcomprises strands of air-textured glass fibre to produce two spacedknitted faces interlinked by yarn which passes from one knitted face tothe other.
 2. A flexible thermal insulation fabric according to claim 1,wherein the fabric is formed by knitting on a double needle bed knittingmachine.
 3. A flexible thermal insulation fabric according to claim 2,wherein the yarn comprises a plurality of strands of glass fibres eachof which is about 1700 decitex.
 4. A flexible thermal insulation fabricaccording to claim 1, having a thermal conductivity, measured in adirection normal to both faces, of the order of 0.10 to 0.20 w/m° k. 5.A flexible thermal insulation fabric according to claim 4, wherein thethermal conductivity is in the range of 0.10 to 0.125 w/m°k.
 6. Aflexible thermal insulation fabric according to claim 1, wherein bothfaces are knitted on the same gauge needles.
 7. A flexible thermalinsulation fabric according to claim 1, wherein one face is knitted onlarger gauge needles than the other face.
 8. A flexible thermalinsulation fabric comprising a double-faced weft knitted structureformed by knitting yarn which comprises strands of air-textured glassfibre to produce two spaced knitted faces interlinked by yarn whichpasses from one knitted face to the other, the yarn comprising tuckstitches which pass from one face to the other face.
 9. A flexiblethermal insulation fabric comprising a double-faced weft knittedstructure formed by knitting yarn which comprises strands ofair-textured glass fibre to produce two spaced knitted faces interlinkedby yarn which passes from one knitted face to the other, wherein atleast some of the glass-fibre is converted to silica.
 10. A flexiblethermal insulation fabric comprising a first fabric according to claim 9joined to a second fabric comprising a double-faced weft knittedstructure formed by knitting yarn which comprises strands ofair-textured glass fibre to produce two spaced knitted faces interlinkedby yarn which passes from one knitted face to the other.
 11. A flexiblethermal insulation fabric comprising a core fabric made of flexiblethermal insulating fabric comprising a double-faced weft knittedstructure formed by knitting yarn which comprises strands of air-textureglass fibre to produce two spaced knitted faces interlinked by yarnwhich passes from one knitted face to the other, and a fabricconstructed in accordance to claim 9 joined to the faces of the corefabric.
 12. A flexible thermal insulation fabric according to claim 9,wherein a finish comprising a vinylacetate ethylene copolymer latex isapplied to one or more surfaces of the fabric.
 13. A flexible thermalinsulation fabric comprising a double-faced weft knitted structureformed by knitting yarn which comprises strands of air-textured glassfibre to produce two spaced knitted faces interlinked by yarn whichpasses from one knitted face to the other, and wherein at least one ofthe faces of the fabric is coated with a refractory material.
 14. Amethod of making a flexible thermal insulation fabric comprising thesteps of weft knitting a double faced glass fibre fabric using yarnwhich comprises strands of air-textured glass fibre on a double needlebed weft knitting machine and interconnecting the faces of the fabricwith at least one linking yarn which passes from one spaced knitted faceto the other.
 15. A method according to claim 14, wherein both faces ofthe fabric are knitted on needles of the same gauge.
 16. A methodaccording to claim 14, wherein a first face of the fabric is knitted onneedles of a larger gauge than that of the needles on which the otherface is knitted.
 17. A method according to claim 16, wherein the needlesof one bed are of 5 gauge and the needles of the other bed are of 2.5gauge.
 18. A method according to claim 14, wherein the fabric is knittedusing yarn which comprises a plurality of strands each of which isapproximately 1700 decitex.
 19. A method of making a flexible thermalinsulation fabric comprising the steps of weft knitting a double facedglass fibre fabric using yarn which comprises strands of air-texturedglass fibre on a double needle bed weft knitting machine andinterconnecting the faces of the fabric with at least one linking yarnwhich passes from one spaced knitted face to the other, wherein the oreach linking yarn is formed by tuck stitches which pass from one face ofthe fabric to the other.
 20. A method according to claim 19, wherein thetuck stitches are formed by wrapping glass fibre threads around selectedneedles of one bed and selected needles of the second bed.
 21. A methodof making a flexible thermal insulation fabric comprising the steps ofweft knitting a double faced glass fibre fabric using yarn whichcomprises strands of air-textured glass fibre on a double needle bedweft knitting machine and interconnecting the faces of the fabric withat least one linking yarn which passes from one spaced knitted face tothe other, wherein the glass fibre fabric is leached by contacting thefabric with hydrochloric acid to convert at least some of the glassfibre to silica.
 22. A method according to claim 21, wherein a finish isapplied to the fabric by contacting the fabric with solution comprising50% by weight vinylacetate ethylene copolymer latex and an aqueoussilicone elastomer.
 23. A method of making a flexible thermal insulationfabric comprising the steps of weft knitting a double faced glass fibrefabric using yarn which comprises strands of air-textured glass fibre ona double needle bed weft knitting machine and interconnecting the facesof the fabric with at least one linking yarn which passes from onespaced knitted face to the other, wherein a first face of the fabric isknitted on needles of one bed which are of larger gauge than the needlesof the other bed, using a yarn which is thicker than the yarn used forknitting the second face.